1. Field of the Invention
This invention relates to polyester compositions containing a brominated flameproofing agent. More especially, this invention relates to an improved polyester composition containing a brominated flameproofing agent wherein the composition contains an epoxy resin. In accordance with the invention, there has been the discovery that by the use of an epoxy resin in such a composition, decomposition behavior of the composition is markedly improved. By improvement of such decomposition behavior, determined by measuring the reduced viscosity of the resultant composition in various forms, there is provided a polyester composition whose end use properties are materially improved. The polyester compositions of the invention include in particular polybutylene terephthalate (PBT) polyester compositions, especially those containing as brominated flameproofing agent a brominated poly-p-xylyleneglycol-bisacrylate, e.g., polytetrabromo-p-xylyleneglycol-bisacrylate (TBX-PA).
2. Discussion of the Prior Art
The properties of use of polyesters are directly related to their viscosity. The higher the viscosity, the better the rigidity, strength and toughness of the molded articles. In a separate postcondensation operation in the solid phase, the viscosity obtained in the polycondensation is therefore increased further with attendant improvement of the mechanical properties, which in the case of PBT is of considerable importance.
A measure of the viscosity is the so-called reduced viscosity, .eta. red., based on the formula ##EQU1## where .eta..sub.o is the viscosity of the solvent (60 parts by weight of phenol and 40 parts by weight of tetrachloroethane), .eta. is the viscosity of the solution, and c is the concentration of the solution (1 g/100 cc), measured at 25.degree. C.
In many cases, technically polycondensed PBT, for example, will have .eta. red. values ranging from 0.8 to 0.9 dl/g, which can be increased by postcondensation to 2.0 dl/g and higher, depending on the end use. Commercial PBT often has .eta. red. values ranging from 1.3 to 1.6 dl/g. Such a PBT has good mechanical properties, including good flexural strength, impact resistance, and notch impact strength.
Reinforcements, and particularly glass fibers, are added to a large proportion of the polyesters, and especially of PBT, to increase their rigidity. Here, too, a high viscosity will improve the properties of use of injection-molded articles.
Because of the flammability of polyesters, the flameproofing of unreinforced and reinforced polyesters is of great importance. Finely divided flame-retardant additives can be thermoplastically worked into PBT and other polyester granules, optionally together with glass fibers, on a kneader or extruder, for example. This must be done at temperatures ranging from 240.degree. to 300.degree. C. The thermoplastic mixture is extruded into a strand, for example, and the latter is chopped into granules, from which articles are then molded on an injection-molding machine, the processing temperatures employed again ranging from 240.degree. to 300.degree. C.
However, the necessarily high processing temperatures result in decomposition of the polyesters with attendant reduction of the viscosity. Since the best polyesters have the highest viscosity and therefore require the highest processing temperatures, it is precisely the highest-grade polyesters which suffer the most damage through a reduction of the viscosity.
Many additives to the thermoplastic mixtures contribute to a reduction of the viscosity. For example, many lubricants and colorants, and almost all fillers and reinforcements, will intensify the reduction in viscosity. Because they lower the viscosity, the fillers added with a view to improving the properties of the material do not achieve their full effect. The reinforcing effect of glass fibers is also lessened by the reduction in viscosity.
The decomposition of polyesters sets in already during processing in the extruder and is increased by the heavier thermal stresses set up during injection molding. Now in the case of molded articles incorporating flame retardants, such decomposition is particularly deleterious and serious since many flame retardants and their synergists themselves have an adverse effect on the mechanical properties of the molded articles and the reduction in viscosity will further increase embrittlement and may in the end render the molded articles unfit for use.
The simultaneous presence of a plurality of substances promoting decomposition results in enhanced and often additive decomposition effects. Decomposition is particularly pronounced when both reinforcements such as glass fibers and flame retardants are present.
This process is totally undesired since it is directly responsible for an impairment of the properties of use of molded articles so produced. It has therefore been sought, in the case of PBT and other polyesters, to start out with as high a viscosity as possible in order to still have a sufficiently high residual viscosity, and hence adequate properties of use, in the finished molded article. However, limits are imposed on this approach by these three factors:
1. The higher the initial viscosity, the more pronounced the decomposition in thermoplastic processing.
2. Polyesters of very high viscosity are more difficult to process thermoplastically.
3. Postcondensation of polyesters up to very high viscosities reduces the economy of the process.
It has been known from German patent application DOS No. 25 01 988 to add epoxides to both reinforced and unreinforced polyesters so that when hardening and crosslinking agents, referred to as catalysts, are added at the same time in amounts of from 0.03 to 0.5% of the total mixture a lengthening of the chain of the polyester is achieved, and with it compensation for the decomposition.
However, the crosslinking agents have been found to give rise to discoloration, and occasionally embrittlement, at the high processing temperatures of the polyesters.
It has also been known to process similar polyesters incorporating epoxides with certain flame retardants in accordance with German patent application DOS No. 25 02 835, use being made of the aromatic monomeric bromine compounds decabromo diphenyl oxide, hexabromo or octabromo diphenyl together with Sb.sub.2 O.sub.3. While such monomeric flame retardants do not result in particularly pronounced decomposition of polyesters, they are not especially compatible with polyesters and will migrate from molded articles, leaving a coating. These flame retardants will volatilize quickly, especially at elevated temperatures of use, and the original flammability therefore will soon be restored.
On the other hand, polyesters incorporating flame retardants are known from German Pat. No. 25 27 803 and U.S. Pat. No. 4,128,709 whose polymeric flame retardants do not migrate and whose flame retardancy is permanently preserved even at high temperatures of use.